Angiotensin II-binding protein in adult and neonatal rat heart

An angiotensin II-binding activity has been identified in the 100,000 x g supernatant fraction of adrenal gland, kidney, liver, heart and brain of adult rat. The binding is specific for angiotensin II; it is of high affinity and completely dependent upon the presence of an organomercurial, p-chloromercuriphenylsulfonic acid. Reducing agents, on the other hand, cause a dissociation of bound ligand. Covalent cross-linking of [125I]-angiotensin II to the soluble fraction from rat heart followed by SDS-polyacrylamide gel electrophoresis and autoradiography indicated that the macromolecule that binds angiotensin II is most probably a protein with an apparent mass of 78,000 dalton. A comparison of the binding of angiotensin II to the 100,000 x g supernatant fraction from both neonatal (1-3-day-old) and adult (3-month-old) rat hearts revealed that angiotensin II binds with similar affinity and specificity, but the number of binding sites is 3-fold higher in the neonatal heart (KD and Bmax were 10.4 +/- 3.1 nM and 1.6 +/- 0.4 pmol/mg protein for adult and 8.8 +/- 2.9 nM and 4.9 +/- 0.7 pmol/mg protein for neonatal heart, respectively). The membrane fraction prepared from neonatal rat heart similarly bound angiotensin II in a saturable manner and with high affinity (KD 4.3 +/- 0.5 nM and Bmax 146.4 +/- 4.9 fmol/mg protein), but a similar membrane fraction prepared from adult rat heart failed to show any angiotensin II binding. These observations indicated that, in rat heart, there is a decrease of angiotensin II-binding sites, both soluble and membrane bound, with age. Hence, rat heart, at various stages of development, and myocytes prepared from it should provide a suitable system with which to study the developmental regulation of the angiotensin II-binding protein.